A brief history of the many attempts during the 19th Century to build a tunnel under the Thames River in London:
Another bridge was out of the question—it would deny sailing ships access to the Pool of London—and ambitious men turned their thoughts to driving a tunnel beneath the Thames instead. This was not such an obvious idea as it might appear. Although demand for coal was growing fast as the industrial revolution hit high gear, working methods remained primitive. Tunnels were dug by men wielding picks in sputtering candlelight.
No engineers had tunneled under a major river, and the Thames was an especially tricky river. To the north, London was built on a solid bed of clay, ideal tunneling material. To the south and east, however, lay deeper strata of water-bearing sand, gravel and oozing quicksand, all broken up by layers of gravel, silt, petrified trees and the debris of ancient oyster beds. The ground was semi-liquid, and at depth it became highly pressurized, threatening to burst into any construction site.